doc: device-tree-bindings: watchdog: document common properties

Adds simple documentation about common properties for watchdog
in the device tree.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Stefan Roese <sr@denx.de>

doc/device-tree-bindings/watchdog/common.txt

+Common watchdog properties.
+
+Optional properties:
+- timeout-sec : Timeout of the watchdog in seconds
+                If this timeout is not defined, the value of WATCHDOG_TIMEOUT_MSECS will
+                be used instead.
+- hw_margin_ms : Period used to reset the watchdog in ms
+                 If this period is not defined, the default value is 1000.
+- u-boot,noautostart : Specify that this watchdog should not autostart
+                       When the config option WATCHDOG_AUTOSTART is set, all enabled
+                       watchdogs are started. This property allows specifying that this
+                       watchdog should NOT be started.